Half-life symbol

A t with subscript one-half, the time for half a quantity to decay.

Also known as: Half life, t with a half underneath

ScientificMathematicalStandard profile

Meanings

t½ is the time for half of a decaying quantity to remain. It is constant regardless of how much you start with, which is the surprising property that makes radiometric dating possible.

Common modern usageVerified fact
Denotes the time required for half of a decaying quantity to remain, whether of radioactive nuclei or of a drug in the body.
Contexts: Mathematics, Laboratory
Common modern usageVerified fact
Is independent of the starting amount, which is what makes it a property of the substance rather than of the sample — and what makes radiometric dating possible.
Contexts: Mathematics, Laboratory
Common modern usageVerified fact
Relates to the decay constant by a fixed factor, so a source stating one implicitly states the other.
Contexts: Mathematics, Laboratory

Visual description

A lower-case italic letter t followed by a subscript fraction one-half, its numeral and divider set below the baseline.

Symmetry: none
Orientation: upright

Used alongside: Becquerel symbol
Activity and half-life are the two ways a source's decay is described.

Confidence

IdentityHigh confidence: High confidence because the identity is documented in encyclopedic references.
Historical originModerate confidence: Moderate confidence because the historical origin is documented in encyclopedic references, but early or supporting evidence is limited, but the regional or cultural match to this specific claim is not precise.
Current useHigh confidence: High confidence because the current use is documented in encyclopedic references.
Visual matchHigh confidence: High confidence because the visual match is documented in encyclopedic references.

Sources

Wavelength
Wikipedia · retrieved 2026-08-01
Tertiary reference. Used for what a symbol denotes in ordinary scientific practice and for which fields reuse it, never for a claim that any body assigns the letter its meaning.
Half-life
Wikipedia · retrieved 2026-08-01
Tertiary reference. Used for what a symbol denotes in ordinary scientific practice and for which fields reuse it, never for a claim that any body assigns the letter its meaning.

Last reviewed 2026-07-31

Part of

Physical constants and quantity symbols

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Meanings

t½ is the time for half of a decaying quantity to remain. It is constant regardless of how much you start with, which is the surprising property that makes radiometric dating possible.

Common modern usageVerified fact

Denotes the time required for half of a decaying quantity to remain, whether of radioactive nuclei or of a drug in the body.

Contexts: Mathematics, Laboratory

Common modern usageVerified fact

Is independent of the starting amount, which is what makes it a property of the substance rather than of the sample — and what makes radiometric dating possible.

Contexts: Mathematics, Laboratory

Common modern usageVerified fact

Relates to the decay constant by a fixed factor, so a source stating one implicitly states the other.

Contexts: Mathematics, Laboratory

Confidence

IdentityHigh confidence: High confidence because the identity is documented in encyclopedic references.
Historical originModerate confidence: Moderate confidence because the historical origin is documented in encyclopedic references, but early or supporting evidence is limited, but the regional or cultural match to this specific claim is not precise.
Current useHigh confidence: High confidence because the current use is documented in encyclopedic references.
Visual matchHigh confidence: High confidence because the visual match is documented in encyclopedic references.

Sources

Wavelength
Wikipedia · retrieved 2026-08-01
Tertiary reference. Used for what a symbol denotes in ordinary scientific practice and for which fields reuse it, never for a claim that any body assigns the letter its meaning.
Half-life
Wikipedia · retrieved 2026-08-01
Tertiary reference. Used for what a symbol denotes in ordinary scientific practice and for which fields reuse it, never for a claim that any body assigns the letter its meaning.

Last reviewed 2026-07-31

Meanings

Visual description

Related & commonly confused symbols

Meanings

Visual description

Related & commonly confused symbols